<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>

    <!-- 找出数组中重复出现的元素 -->

    <script>
        var arr = [1, 2, 3, 4, 5, 6, 7, 2, 4];
        var sava = [];
        for (var i = 0; i < arr.length; i++) {
            for (var j = i + 1; j < arr.length; j++) {
                if (arr[i] === arr[j] && sava.indexOf(arr[j] === -1)) {
                    sava.push(arr[i])
                }
            }
        }
        console.log(sava);
    </script>
</body>

</html>